Tìm x biết x + 2x + 3x + 4x + . . . + 2019x = 2020

Câu 1 : Tìm x biết : 
a ) x + 2x + 3x + 4x +  . . . + 2019x = 2020 . 2021
b )$\frac{x - 1 }{2021}+ \frac{x - 2}{2020} - \frac{x - 3 }{2019} = \frac{x - 4 }{2018}$​
$c) 3^{x - 1 } + 5.3^{x - 1 }= 162$​